The present invention relates to ion cyclotron resonance spectroscopy.
Ion cyclotron resonance is a well known phenomenon and provides a sensitive and versatile means for detecting gaseous ions. According to this phenomenon, a gaseous ion moving in a static magnetic field is constrained to move in a circular orbit in a plane perpendicular to the direction of the magnetic field, and its movement is unrestrained in directions parallel to the magnetic field. The frequency of this circular motion is directly dependent upon the strength of the magnetic field and the mass-to-charge ratio of the ion. When such orbiting ions are subjected to an oscillating electric field disposed orthogonally to the magnetic field, those ions having an orbital frequency equal to the frequency of the oscillating electric field absorb energy from the electric field and are accelerated to larger orbital radii and higher kinetic energy levels.
This phenomenon has been used by ion cyclotron resonance mass spectrometers to detect different types of ions. An example of this type of mass spectrometer and its operation are described in U.S. Pat. No. 3,937,955. This device includes a single cell ion trap formed by a six-electrode cube mounted within a high-vacuum chamber. An electron gun directs a pulsed beam of electrons through the cell, which ionizes a sample of a gaseous material to the analyzed. The bias potential on two electrodes perpendicular to the magnetic field traps the charged ions within the cell.
An externally applied magnetic field causes the ions to move in circular orbits in planes that are perpendicular to the direction of the field. Each ion has an angular cyclotron frequency .omega..sub.c given by the equation: EQU .omega..sub.c =(q/m)B (1)
(q/m) is the mass-to-charge ratio of the ion, and B is the magnetic field strength. A typical sample being analyzed consists of several types of ions which orbit (cyclotron) at different frequencies due to their different mass-to-charge ratios.
Following the formation of ions, the cyclotron orbital radii is increased by applying a time-varying excitation potential to one or two opposed cell electrodes which are parallel to the magnetic field. This produces an electric field that excites the orbiting (cyclotroning) ions to higher kinetic energy. After the excitation pulse, the orbiting ions induce an alternating voltage across another pair of opposed electrodes which are positioned parallel to the magnetic field direction. This voltage has a frequency produced by the superposition of signals at each of the ion cyclotron resonant frequencies. The amplitude of each component signal is proportional to the number of ions having the corresponding mass-to-charge ratio. The composite signal produced by the excited ions typically is amplified, digitized and stored in a computer memory. Fourier analysis is employed to transform the composite signal in the time domain into a frequency domain signal containing information regarding the mass and relative abundance of each type of ion within the cell.
The ability of the conventional Fourier transform analysis ion cyclotron resonance spectroscopy in distinguishing between different ions having similar mass-to-charge ratios (i.e. closely separated resonant frequencies) is directly related to the period during which the time domain signal from the cell is sampled. For example, if two different ions resonate at frequencies which are one hertz apart, the cell's output signal will have to be sampled for one second or a hundred times longer than if the frequencies are separated by 100 hertz. The ability to resolve close resonant frequencies is important if the analysis is to detect different ions of the same nominal mass, for example.
However, as the composite signal sampling time increases, so do the artifacts in the signal due to inhomogeneities in the magnetic and electric fields within the ion trapping cell. Additionally, signals from different ions have different durations. Such artifacts can greatly affect the quantitative measurement of each type of ion in the sample. In other terms, the longer the required sampling period, the greater the inaccuracy in the data. As a consequence, it would be desirable to be able to perform a high resolution transformation of the composite signal from the time to frequency domains using a short signal sampling period during which the effects of the previously mentioned artifacts will be small.